1. Technical Field
The invention relates generally to semiconductor cleaning, more particularly to a method of electrochemical removal of contaminants, including solid particles, organic contaminants, and metal ions, from the surface of a silicon wafer during IC chip manufacture.
2. Background of the Invention
The density of ultra large-scale integrated circuits (ULSI) is rapidly increasing and the device feature size is becoming smaller. This leads to more stringent requirements for chip surface contamination. In the process of producing ULSI, the surface cleanliness of the chip is one of the most important factors influencing the quality and reliability of the devices.
In the preparation process, the chip's surface may be exposed to various contaminants, such as oil, wax, photo resist, metal ions, and many others. These contaminants must be removed; otherwise, they will have adverse impact on the follow-up processes and could lead to catastrophic chip failure.
Chip surface contaminants may weaken the bonding between the chip surface and the deposition membrane, and thereby adversely affect IC properties. They may also be converted to other harmful products, causing additional pollution on the chip surface. For example, when a chip is heated at high temperatures in an anaerobic environment, organic contaminants are carbonized, and will react with silicon to produce silicon carbide on the chip's surface.
Conventional solutions used for removal of organic contaminations from the IC chip surface include RCA-I and RCA-III. RCA-I, also known as AMP, is a mixture of NH4OH and H2O2 in H2O at 65-80° C. It is an alkaline liquid, having oxidative and chelating properties. It can remove certain types of organics, photo resist, membrane remnants, and some heavy metal ions. On the surface of the silicon chip, RCA-I can react with silicon. This helps the organic contaminants break away from the chip surface. However, this also corrupts the silicon surface according to the following chemical equations:Si+2 NH4OH+H2O→(NH4)2SiO3+2 H2Si+2 NH4OH+H2O2→(NH4)2SiO3+2 H2OBecause the reactions take place unevenly throughout the silicon surface, they are a cause of non-planarity. At high pH conditions, H2O2 facilitates the deposition of metal contaminants on silicon surface. Even when the concentration of H2O2 is too low to affect deposition, NH4OH will corrupt the silicon surface, and silicon flecks will appear. When working with NH4OH appropriate safety precautions must be taken which increases the production cost.
RCA-III, also known as SPM, is a mixture of H2SO4 and H2O2 in deionized H2O at 100-130° C. RCA-III is an oxidative acid solution. It can remove organic matter, photo resist, and other acid soluble impurities. However, because RCA-III contains H2SO4, a strong acid and an oxidant, it carbonizes organic impurities, causing the silicon surface to turn black and making the cleaning process more difficult. H2SO4 in the wastewater pollutes the environment.
The aforementioned cleaning solutions contain chemicals at a certain fixed concentration. Because the concentration of contaminant on the silicon surface varies, the chemical reactions between the cleaning chemicals and the contaminants are not optimized and may not even occur at too low concentrations. In addition, if cleaning solutions are reused, their cleaning power will decrease over time as the cleaning chemicals are consumed. To maintain a lasting cleaning effect, frequent replacement of cleaning agents is needed. This often generates a large amount of wastewater and leads to environmental pollution.
Another reagent used for removing organic contaminants from chip surfaces is aqueous O3. O3 is a powerful water-soluble oxidant. However, the solubility of O3 in water is not easily controlled. In addition, stringent safety precaution must be taken when working with O3 because it is hazardous to human health.
Solid particles and metal ions have a great impact on the quality and reliability of IC chips. Metal ion adsorption on the substrate surface may disrupt silicon doping of the underlying structures resulting in reduced lifetime, channel breakdown and interface defects. Solid particles adsorbed on the surface will cause about pinholes and tiny cracks resulting in short circuit or open circuit of adjacent wires.
In order to meet the requirements for further development of microelectronics, a method for removal of contaminants should remove not only organic contaminants but also solid particles, metal ions, etc. However, conventional methods of chip surface cleaning remove organic contaminants only. Accordingly, much opportunity remains for improvement in this area of technology.